1. Field of the Invention
This invention relates to the preparation of polymer foams and is more particularly concerned with polyisocyanurate foams.
2. Description of the Prior Art
Cellular polymers in which the major recurring polymer unit is isocyanurate, are well known and widely used in the art; see for example U.S. Pat. Nos. 3,516,950, 3,625,872, 3,644,232, 3,676,380, 3,725,319, 3,745,133, 3,763,057, 3,793,236, 3,799,896, 3,849,349, 3,876,568, 3,891,579, 3,909,465, 4,025,469, 4,039,487, and 4,092,276. Generally speaking, the polyisocyanurate foams prepared in accordance with the prior art are found to be more fire resistant and show lower smoke evolution on burning, in comparison to polyurethane foams, when subjected to the various testing procedures known and used in the foam art. Polyisocyanurate foams tend to be brittle or friable and to overcome this deficiency the prior art teaches that various types of polyols added in varying amounts will lower foam friability. But this is not without its disadvantages because the fire and smoke properties of the polyisocyanurate foam deteriorate as the polyurethane content increases. Therefore a rather fine balance exists between the amount and type of polyol one adds to a polyisocyanurate foam formulation in order to maintain maximum flame and smoke resistance while at the same time reach an improvement in foam friability.
Polyester polyols of many types are well known as useful polyol additives in polyisocyanurate formulations; see the patents cited supra. Typically, U.S. Pat. No. 4,039,487 discloses the use of certain aromatic polyester polyols in particular proportions to prepare polyisocyanurate foams possessed of remarkably good fire resistance and low smoke evolution upon combustion in the absence of fire retardants or smoke inhibiting agents. However the friability of the foams obtained tends to be relatively high. Furthermore, the polyols are comparatively expensive to manufacture.
U.S. Pat. No. 4,092,276 discloses the use of certain aromatic polyester polyols in preparing polyisocyanurate foams possessed of good friability and good fire resistance but not very strong in compressive strength. The polyols disclosed therein, like the ones referred to above, require the use of expensive ingredients to manufacture.
A further problem which arises from the use of aromatic polyester polyols, particularly those of low molecular weight (which includes the ones referred to above), is the tendency for the polyols themselves to be solid at room temperature, or, at best, to be possessed of very high viscosity and poor solubility in resin mixtures, which properties makes them difficult to handle in resin formulations.
We have now found, quite unexpectedly, that polyisocyanurate foams prepared with a minor amount of a cheap, readily available, by-product type of polyol mixture are characterized by a high degree of fire resistance with low smoke evolution on combustion and a surprisingly low foam friability. Further, the polyol mixture is in liquid form and therefore handled easily. Combined with these advantageous properties are the improved compressive strengths of the foams of the present invention over those found in the prior art. Additionally, and most surprisingly, the instant foams upon combustion form a protective intumescent char over unburnt foam and show very low heat and smoke release when tested in accordance with the Smith test defined below. This combination of advantageous properties has not heretofore been disclosed in the art.